Teacher's Notes For Pressure

TEACHER'S NOTES FOR LEARNING-BY-DOING LEAFLETS ON PRESSURE
AND WATER TECHNOLOGIES

The Learning-By-Doing leaflets in this pack are divided into two
setions (I) Pressure and (II) Water Technologies. The latter covers the
following topics: water supply; water-lifting devices and water treatment

TEACHER'S NOTES FOR (I) PRESSURE

LEAFLET NO. 37: UNDER PRESSURE

TEACHER'S TIPS

Air pressure activities

There are many more air pressure activities you and your
students can try. Here are some examples, but your students may be able to think
up other air pressure "tricks" to amaze people:

(1) The magic glass

You will need:

* a glass;* some water;* a piece of
cardboard.

Fill the glass to the brim with water. Place the cardboard over
the glass. Hold the cardboard against the glass, and turn the glass upside down.
Take your hand away from the cardboard:

Figure

What happens? If you have done everything properly, the card
will stay in place.

Conclusion: The pressure of the outside air acts against the
card, and forces it against the glass, because it is stronger than the pressure
of the water. (If it doesn't work the first time, keep trying!)

(2) Blowing strips

You will need:

* 2 long strips of paper

Hold the two strips of paper a few centimetres (about 2 inches)
apart, dangle them in front ofyou. Ask your friends what they think will
happen if you blow steadily between the strips. They may say "The paper will be
blown apart," but in fact, the strips move together. Why? Because as you blow,
the pressure of air between the strips is lowered, and the pressure of air on
the outside of the strips becomes greater than the air between- the strips, so
the pieces are pushed together.

(3) Jabbing a potato

You will need:

* a drinking straw;* a potato.

Hold the straw on its sides. Try jabbing it into the potato.
Then place the index finger over one end of the straw. Rapidly jab the straw
through the potato, being careful to hit the potato squarely with the straw. How
can a straw go through the potato?

Figure

By sealing the top end of the straw with a finger, the air in
the straw is trapped when the other end strikes the potato. Air exerts
considerable pressure under normal conditions. But when it is compressed, the
pressure is increased. The compressed air in the straw gives the straw strength
enough to prevent its bending. The fragile straw will easily go through the
potato.

Learning by trial and error

The students may have some difficulty getting the activities to
work. This is an excellent opportunity for them to act like science detectives
and try and track down that small detail that means the difference between
success and failure in the experiment. For example, if students have trouble
with the magic glass activity above, which involves holding a piece of cardboard
under a glass of water, it may be because of the cardboard. As long as the
cardboard does not sag or get soggy, it will stay in place by itself. If the
cardboard is not firm and flat to begin with, it will let air in and water out,
so the experiment will not work.

Stoppers

In the experiment "Lifting water with air pressure", a stopper
is needed as part of the experiment equipment. Stoppers can be made from a
variety of things, such as a sponge rubber ball, a cork, a rubber stopper or a
plastic bag:

Figure

Figure

Air pressure in action

Find examples in your local environment that demonstrate air
pressure in action. Here are two possibilities:

(1) Birds' flight: Have you ever wondered how birds fly? They
flap their wings, but how does this keep them up in the air? They don't come
crashing down because they push on the air with each wing-beat, and the air
pushes back. The force of the wing-beat makes more pressure in the air
underneath the wing, and his extra pressure pushes the bird up.

Ask the students to watch a bird in flight. See how the wing
going up does not push as hard against the air as when the wing goes down. It's
similar to the way people tread water.

Figure

(2) Bicycle pumps: If you squash air, or any other gas, into a
smaller space, it pushes back. It is trying to spread out again and fill the
space it used to have before you compressed it. This pushing back is very useful
because it makes the air act like a spring. If you block the end of a bicycle
pump with your finger, then push the handle hard, you will feel the springiness
of the compressed air inside the pump.

Measuring air pressure

Atmospheric pressure can be measured with a rubber suction cup.
The force required to pull the sucker away from a smooth surface can be found by
using a spring balance:

Figure

The area on which the atmospheric pressure is acting can be
measured by pressing the sucker on a piece of squared paper. It is best to use a
sucker which has a hook attachment. If one of this sort is not available, tie
some copper wire firmly around the neck of the sucker to form a loop. If
possible try this experiment using different sized suckers.

All three impressions were made with approximately the same
amount of force (i.e. the book). The deepest imprint is made by the nail point
impression. The shallowest imprint is made by the coin. You can make a deeper
imprint by distributing the force of the push over a smaller area.

(a) To increase pressure either increase the applied
force or decrease the area over which the force is applied.

(b) To decrease pressure, decrease the applied force or increase
the area over which the force is applied.

(c) By lying flat on his stomach, the man is spreading the force
over a large area.

Lifting water with air pressure

(1) As the air inside the jar cools, a partial
vacuum is created inside the jar. Since air pressure outside the jar is greater
than that inside the jar, water is sucked into the jar.

(2) When straw B is blocked, air cannot get into the jar to
replace the liquid being sucked out. It will be difficult to suck on straw A
since there will be less air pressure inside the jar than outside it. If air is
blown into straw B, the air pressure inside the jar will become greater than
that outside the jar. Thus, it will be easier to suck liquid through straw
A.

Variation on this activity: Fill a soda bottle completely with
water. Put a straw in the soda bottle, and seal with modelling clay, cotton, or
wet paper. Challenge a friend to drink the water from the bottle. If air is
completely excluded from the bottle, your friend will not be able to do it.

Make your own barometer

A barometer is a simple instrument to read. As the air pressure
changes, a needle moves around a dial which is marked Rain, Showers etc. so that
we can read out the changes in weather. The learning leaflet describes how to
make one simple type of barometer. Here is another way to make a barometer.

You will need:

* a bottle;* a piece of white tape;* a small
dish;* some water.

Fill the bottle full of water. Cover the bottle with a dish,
hold the dish and bottle tightly and invert them so the bottle stands upside
down in the dish of water. Changes in air pressure will affect the level of
water in the bottle. Stick a small piece of tape on the outside of the bottle,
and mark the water level each day.

Figure

The students could make both types of barometers, and keep daily
records of readings from each one. Then, the students can determine which
barometer works best.

GOING FURTHER

Activity: Making a hosepipe water level

To reduce soil erosion, land should be farmed across the
hillside, not up and down the slope. Better still, barriers such as hedges or
stone lines running level with the contours can help slow run-off and
increase the amount of rainwater that soaks into the ground. Here is one device
that can be used to mark out contour lines on a slope. You will need:

Each end of the hosepipe is attached by string to a stake. The
hosepipe is filled with water. Water is added or tipped out until the water
level lies between the marked lines on the stakes. Provided all air bubbles are
expelled, whenever the bottoms of the two stakes are level, the water at each
end of the hosepipe should come up to the same mark on each of the stakes. Now
the device is ready to be used.

Figure

LEAFLET NO. 38: GETTING LIQUID TO WORK FOR YOU

TEACHER'S TIPS

Experimental Results

The experiments described in the student leaflet use very simple
equipment to demonstrate water pressure at work.

Seeing Water Exert Pressure

The pressure of water above the holes causes the jets of water
to flow from the holes. The water near the bottom of the carton has the force of
all the water above it pushing it out, and so the water shoots out further from
this hole. The water coming out near the top of the carton has very little water
- and therefore pressure - above it.

Water pressure in different sized containers

Pressure is not affected by the size or shape of the vessel: it
only depends upon the depth of the liquid.

Note: The water may squirt out the same distance at first, but
the water level will drop more quickly in the narrower container, so the water
will not continue to squirt out as far as that in the wider container.

The following experiments using more elaborate equipment may be
tried in class:

You will need:

* two lengths of rubber tubing, one long (30 cm) and
one short;* two ballpoint pen tubes or two transparent plastic drinking
straws;* food colouring* a funnel;* two pieces of board;* some
thin rubber - perhaps, from an inner tube* string;* clips;*
ruler,* a bucket of water.

1. Connect the drinking straws to the short length of tubing,
and attach them to an upright board, as shown in the diagram that follows.

2. Put some water in the tube to a depth of 6-8 cm., and add one
or two drops of food colouring so that the water is clearly seen.

Tape a ruler next to the open straw. This is your pressure gauge
or manometer (said: man- OM-eter).

Figure

3. Stretch a piece of thin rubber tightly over the funnel and
secure with some string. Fix the funnel to the manometer with the 30 cm length
of rubber tubing.

4. Push the funnel into the bucket of water. What happens? As
the inverted funnel goes deeper into the water, the manometer liquid level
changes.

Conclusion: Water pressure forces the air in the funnel and
manometer tube to move the manometer liquid.

5. Measure the water pressure when the funnel is held just below
the surface of the water. Then, move the funnel down to the bottom of the
bucket. How does the pressure change with depth?

Extensions:

(A) Try this experiment using different sized
funnels.

(B) Fill a jar of small diameter and ajar of large diameter with
water to the same depth. Measure the pressure at the bottom of each jar. How do
they compare?

(C) Take two jars of the same size and fill one with water and
one with a less dense liquid such as rubbing alcohol. Make sure the depths of
the liquids are the same. Using the funnel and manometer as described in 1-4
above, measure the pressure at the bottom of the water jar and then at the
bottom of the alcohol jar. How do they compare?

Hydraulic machines

A garage and a construction site are two good places to see
hydraulic machines at work. Visits to such places require the owner's (and
parents') permission, and should be undertaken only when students can be well
supervised.

An hydraulic lift in a garage easily raises the great weight of
a car for inspection. Although it contains only one piston, it does work by
hydraulics. Ask the garage owner if it is possible for your students to see the
hydraulic lift in action. It may also be possible to study how a car's hydraulic
brakes work.

Machines such as bulldozers, work with hydraulic rams. Each ram
consists of a piston in a cylinder connected by pipes to a central reservoir of
hydraulic fluid. By having two-way valves in the hydraulic cylinder, the piston
can be made to push in either direction. That is how the powerful rams on a
bulldozer can gently lift and lower its huge steel blade.

Hydraulic Ram

A hydraulic ram is a machine which pumps water up to a height of
100 metres or more. It can be used to pump water from a river in the valley to a
village in the mountains.

The pump uses no motor and is easy to repair, but it does not
work in all places. It needs falling water, so it cannot work in areas which are
very flat and it cannot pump water from a well. The falling water forces a small
amount to a height greater than the source. At least 7-14 times as much water is
needed to work the pump that is raised. A small amount of water with plenty of
fall can pump as much water as a greater amount with only a little fall. The
water source needs to be at least one metre above the pump. Plenty of water must
be available. It is one of the cheapest methods of raising water and it needs
only a little maintenance. The diagram below shows how it works.

Water flows down the drive pipe to the valve box, and at first
returns to the stream through the waste valve. As the water flow increases in
speed, the waste valve closes. The sudden change in the flow down the drive pipe
leads to a rise in the pressure in the valve box. The delivery valve opens and
water is forced up into the air vessel and to the delivery pipe.

* a large plastic bottle from which the bottom has
been removed;* a ball point pen tube;* a one-hole stopper which fits the
bottle top, and through which the ballpoint pen tube fits;* a metal
T-tube;* three pieces of rubber tubing of various lengths.

1. Set up the apparatus as shown:

Figure

2. Fill the bottle with water. (This reservoir will act as a
natural stream flowing downhill.) Pinch the tube at the end. Let the water run
from the end of the tube. Stop the flow suddenly by quickly pinching the tube,
and note the height to which the water squirts from the jet tube. Let the water
flow and stop alternately, and you have a working model of the hydraulic
ram.

In both examples above, the momentum or speed of the water is
important. Similar to the lever examples, the amount of water is exchanged for
the distance it is raised.

GOING FURTHER

Making a hydraulic ram

A hydraulic ram is sometimes used to raise water from a low
level to a higher level. It is operated by a flowing stream of water, and it
pumps a fraction of this flow to a greater height.

Can your students design a hydraulic ram? They may use an actual
small fall in a natural stream to pump a much smaller flow of water to a greater
height. Or they could create their own artificial water reservoir. How can the
flow of water be
controlled?